Method for producing steel sheets having resistance against rust formation

ABSTRACT

A METHOD FOR PRODUCING A STEEL SHEET HAVING A RESISTANCE AGAINST RUST FORMATION COMPRISING APPLYING AN OXIDATION TREATMENT TO A STEEL SHEET CONTAINING NOT MORE THAN 0.15% OR C, NOT MORE THAN 0.15% OF SI, NOT MORE THAN 0.02% OF S, 0.005 TO 0.15% OF P, 0.01 TO 0.25% OF CU, AND AT LEAST ONE FROM 0.3 TO 2.0% OF MN, 0.05 TO 2.0% OF CR, 0.05 TO 2.0% OF AL, 0.01 TO 0.2% OF SB, 0.001 TO 0.20% OF AS AND 0.01 TO 0.20% OF BE, WITH THE BALANCE BEING SUBSTANTIALLY IRON AND UNAVOIDABLE IMPURITIES, WHEREBY A NON-VISIBLE OXIDE FILM COMPOSES OF $-FE2O3 IS FORMED ON THE STEEL SHEET.

United States Patent 3,764,398 METHOD FOR PRODUCING STEEL SHEETS HAV- ING RESISTANCE AGAINST RUST FORMATION Toshiro Nishihara, Hidejiro Asano, Shigeyoshi Maeda, and Yashichi Oyagi, Kitakyushu, Japan, assignors to Nippon Steel Corporation, Tokyo, Japan No Drawing. Filed Aug. 6, 1971, Ser. No. 169,845 Claims priority, application Japan, Aug. 10, 1970, ts/69,848 Int. Cl. C23f 7/04 U.S. Cl. 148-6.14 R 2 Claims ABSTRAQT OF THE DISCLOSURE A method for producing a steel sheet having resistance against rust formation comprising applying an oxidation treatment to a steel sheet containing not more than 0.15% of C, not more than 0.15% of Si, not more than 0.02% of S, 0.005 to 0.15% of P, 0.01 to 0.25% of Cu, and at least one from 0.3 to 2.0% of Mn, 0.05 to 2.0% of Cr, 0.05 to 2.0% of Al, 0.01 to 0.2% of Sb, 0.001 to 0.20% of As and 0.01 to 0.20% of Bi, with the balance being substantially iron and unavoidable impurities, whereby a non-visible oxide film composed of y-Fe O is formed on the steel sheet.

The present invention relates to a method for producing steel sheets having resistance against rust formation.

In general, carbon steels (ordinary steels), when exposed to the air, form more or less rust (corrosion) on their surface. Conventional methods for preventing such rust forming on the steels are such that corrosion resistance of steels is improved by alloying with corrosion resistant metals such as Cr and Ni to render the steels noble as in stainless steels, or a dense corrosion layer is formed on the steel surface to prevent further corrosion as in weather resistant steels.

Also in the case of carbon steels, rusts which are formed in a very short time as formed during the steel sheet production or in the transportation of the steel sheets, or rusts which are formed in a relatively long period as formed on edge portions of food cans during their storage are problems. Particularly, the rust problem in cans is increasing as tin free steels (TFS) are being more and more substituted for tin plates as can materials in recent days. Namely, seam or double seam portions of cans, cut edges of crown caps, and scored portions of recently developed easy-open cans are particularly problemsome because they are very susceptible to rust formation due to partial exposure of their non-coated surface, and rust flows therefrom to stain the whole appearance.

In case of conventional tin plates, the problem is less critical because the tin flows into the cut portion and serves to protect the exposed metal surface against rust formation.

However, in the case of tin-free steels such as chromium plated steel sheets, such protection is not expected and the problem is troublesome, and has hindered the application of tin-free steel sheets.

Particularly in the field of can materials, general tendencies are towards the non-use of tin and elimination of surface treatments which are disadvantageous in production cost while these tendencies are supported by improvements of paints and lacquers.

Thus importance has been put on a method for pre venting rust formation on metal surfaces exposed by workings such as the seam or double seam portions, cut edges and scores of easy-open cans by giving corrosion resistance to the material itself and at the same time by applying a simple surface treatment.

In the case of food cans and so on, their storage period is limited by the considerations on sanitation, and it is not necessary to prevent rust formation (permanently), but it is considered to be enough to prevent rust formation for a certain period of time.

The present inventors have conducted extensive studies in order to develop a method for preventing rust which is formed in a very short time or rust which is formed in a relatively long time as mentioned above and to develop a method for producing steel sheets having resistance to rust formation without sacrifice of their workabilities at far less cost than conventional corrosion resistant steels. We have also found that a steel sheet having resistance against rust formation can be obtained by applying an electrolytic oxidation treatment or a chemical oxidation treatment to a carbon steel sheet containing one or more of Mn, Sb, As, Bi, Cr and Al in a very small amount.

Therefore, the object of the present invention is to provide a method for producing steel sheets having resistance against rust formation by a simple treatment.

The feature of the present invention is that an oxidation treatment such as an electrolytic oxidation treatment and a chemical oxidation is applied to a carbon steel comprising one or more of not more than 2.0% of Mn; 0.01 to 0.20% of Sb; 0.01 to 0.20% of As; 0.01 to 0.20% of Bi; not more than 2.0% of Cr, with the balance being substantially iron and unavoidable impurities.

One of the features of the present invention is that an oxide film is formed on the steel sheet by an oxidation treatment. But such oxide film is formed by nature in the air without the oxidation treatment. However, the natural oxide film is composed mainly of 'y-Fe O and its thickness ranges from 10 to 30 A., accompanied with many defects such as the presence of pores and irregular film thickness and thus poor resistance against rust formation. And when the natural oxide film is heated in the air, the film grows and the film construction is changed to form a two-layer film; a film of R 0, near the matrix surface and a film of u-Fe O outside the film of Fe O Through experiments it has been found that such a natural film does not improve resistance against rust formation.

On the other hand, it is also well known that a 'y-F6 O film is mostly formed when the surface of iron is electrolytically or chemically oxidized, but it has been found that when iron or steel which does not contain special elements such as Mn and Sb as in the present invention is subjected to an oxide treatment to form a -y-Fe O film and is exposed in the air, no improved or only very little improved resistance against rust formation is shown, thus no practical value is attained.

Therefore, in the present invention the rust preventive oxide film on the surface of the steel sheets is obtained not by the natural oxide film formation nor merely by the electrolytical or chemical oxidation treatment, but is obtained by the combination of the steel composition containing one or more of Mn, Sb, As and Bi in a certain.

amount and the electrolytic or chemical oxidation treatment applied to the steel, and thus obtained oxide film ('y-F6 O film) is stabilized and dense and thus a steel surface having remarkable resistance against rust formation can be obtained.

The present invention will be described in more detail.

The special elements composing the steel used in the present invention are one or more of Mn, Sb, As, Bi, Cr, and Al. Each of these elements give the steel surface a non-visible stabilized and dense oxide film having good resistance against rust formation. Particularly effective for densing and stabilizing a film having a thickness to 50 A. composed of spinnel type oxide mainly composed of -,-Fe,o and for assuring the above effect, Mn, Cr and Al each in an amount more than as usually contained in steels and Sb, As, Bi each in an amount more than 0.01% are required. However, excessive amounts of these elements do not improve substantially the resistance against rust formation, but cause embrittlement of the material, and particularly excessive contents of Mn, Cr and Al colours the steel surface (interference colour) and thus should be avoided from the viewpoint of appearance.

Therefore in the present invention, the upper limits of Sb, As and Bi are set as 0.20% and the upper limits of Mn, Cr and Al are set as 2.0%. Namely, when Cr, Al or Mn is contained in a large amount, the film is an excessively oxidized state and thus film is coloured (becomes visible), wh'ich deteriorates the commercial value of the product. Regarding Mn, which is usually contained in steels, similar results will be produced when it is contained in combination with Cr and Al, and thus the total amount of Mn+Cr+Al should be not more than 2.0%. However, less than 0.05% of Cr, less than 0.3% of Mn or less than 0.05% of Al is not effective for producing a desired film.

Other elements such as C, Si, P, S and Cu should be in the steel desirably in the following range.

C: Carbon is necessary only for assuring the required mechanical properties and it is desirable to restrict the carbon content to not more than 0.15% to assure resistance against rust formation.

Si: Silicon has adverse effects both on mechanical properties and resistance against rust formation, and it is necessary to limit the silicon content as low a value as possible, desirably less than 0.15

P: Phosphorus is not particularly elfective in combination of the oxidation treatment, but is generally effective to give resistance against rust formation a certain range of content. However, a large content will embrittle the material and thus the phosphorus content is limited to the range of 0.005 to 0.15%.

S: Similarly as silicon, sulfur is usually contained in steels, but it has adverse effects both on the mechanical properties and resistance against rust formation. In order to prevent the adverse effects on the mechanical properties by sulfur it is well known to add manganese. But manganese must be present as a solid solution which does not combine with sulfur in order for the manganese to be effective for resisting rust formation in combination with the oxidation treatment. From the above point, it is desirable to limit the sulfur content as low as possible, desirably less than 0.020%

Cu: In the same Way as phosphorus, copper is not effective for resisting rust formation in combination with the oxidation treatment, but it is effective by itself when added in a ceratin amount. However, a large amount of copper has adverse effects and it is desirable to limit the copper content to the range of 0.05 to 0.25%

Now the steel sheet containing the above elements in the above ranges are obtained by an ordinary method comprising steel making, cogging, hot rolling, cold rolling, annealing and, if necessary, temper rolling.

As for the oxidation treatment for forming the oxide film on the steel sheet surface in the present invention, anodic oxidation in an electrolyte (electrolytic oxidation), or an immersion treatment in a solution of oxidizing agent (chemical oxidation) may be used. Other oxidation treatments such as an immersion treatment in canstic soda (alkali black colouring), a fused nitrate process,

super-heated water vapour process and oxidation by heating in the air are not used in the present invention. The immersion treatment in caustic soda, the fused nitrate process and the super-heated water vapour process are directed to formation of a thick magnetite layer of several microns on the steel sheet surface and blacken the sheet surface as they are commonly used as a blacking method. These treatments are undesirable for the present invention. Although the oxidation by heating in air is the most simple method for an oxide film formation, it does not improve, but rather deteriorates the resistance against rust formation, and particularly when heated above 250 C., the film is coloured by bluing, and thus this method is not desirable. It has been found that only the electrolytic oxidation or chemical oxidation is effective for the steel sheet containing the special elements such as Mn, As and Cr, in order to avoid the difficulties as mentioned above.

The electrolytic oxidation treatment for improving the rust formation resistance in combination with the elements such as Mn, As, Bi, Cr and Al as mentioned above does not require any special consideration, and an ordinary oxidation method can be used. Namely, as for the electrolyte, any electrolyte which gives electric conductivity to the liquid may be used, whether they are acidic, neutral or alkaline. However an electrolyte containing halogen ions such as Clor SO;- which likely destroy the passivated film should be avoided. The most desirable electrolyte includes a low concentration solution of nitrate, sodium borate, sodium carbonate, sodium altho-silicate, caustic soda etc. The anodic treatment is conducted for several minutes to several ten of minutes using the steel sheet as the anode and a suitable material as the cathode. As for the anodic treatment, either a constant potential method or a constant current method may be used. In the constant potential method, it is desirable to set the potential in the passivation range which depends on the type of the electrolyte solution, and in the constant current method it is necessary to select an appropriate current density depending on the type of the solution.

Next regarding the chemical oxidation treatment, effective and desirable results can be obtained by the chemical oxidation treatment alone, but its effective range is limited as compared with the electrolytic oxidation treatment. Commonly used oxidizing agents may be used, and chromic acid, potassium dichromate, potassium permanganate, etc. are particularly useful. Desirable treating conditions are: a concentration of the solution between 3 to 10%, a treating temperature between the ordinary temperatures and C., and a treating time between several seconds and several tens of seconds.

As described above, in the present invention, a dense and stabilized thin non-visible oxide film is formed on the steel sheet surface by adding a very small amount of the special elements as contrasted to the conventional techniques.

If the oxide film is formed in a super oxidation state, the film is coloured and the appearance of the steel sheet is damaged. Thus, it is desirable to form the film in a thickness range of 380 A. to 200 A. According to the present invention, the period before rust formation can be remarkably prolonged and it is possible to keep the steel sheet from rust formation under certain service conditions and periods.

Next, examples of the present invention will be described.

Table 1 shows the rust formation ratios before and after the oxidation treatment of the steel sheets of the present invention in comparison with those of conventional comparative materials.

As clearly understood from Table 1, the rust formation ratio of the present inventive steel sheets is remarkably low as compared with that of the comparative materials and thus the present inventive steel sheets has an excellent rust formation resistance.

TABLE 1 Rust formation resistance (rust formation percent) Before After Conditions of oxidation oxidation oxidation Si Mn P S Cu Sb. As Bi Cr Al treatments treatment treatment 1 0.05 0.01 0.25 0.015 0. 015 0.08 0.05 Electrolytic oxidation with the 35 2 steel sheet as anode stainless steel as cathode in sodium borate boric acid bufiered solution (pH 8.4) at constant potential of +0.6 v. for 30 min. Comparativematerial..- 0.05 0.01 0.25 0. 015 0. 015 do 100 90 2 0.07 0.02 0.29 0.011 0.010 0.05 0.10 Electrolytic oxidation, in 3% 45 sodium altho silicate solution at AJdm. for 5 min. Comparativematerial-.. 0.07 0.02 0.29 0.011 0. 010 0.05 .do 100 95 3 0.07 0.02 0.29 0.013 0.011 0.08 0.05 Electroyltic oxidation, in 5% 40 6 sodium carbonate solution at constant potential of +0.6 v. for min. Comparativematerial... 0.07 0.02 0.29 0.013 0.011 0.08 .do 100 100 4 0.05 0.01 1.0 0.015 0.015 0.08 SameasExampiel 100 2.2 Comparativematerial.-. 0.05 0.01 0.25 0.015 0.015 0.08 100 90 5 0.07 0.02 0.27 0.010 0. SameasExample2 85 5 Comparativematerial... 0.07 0.02 0.27 0.010 0. -do 100 95 6 0.05 0.01 0.21 0.015 0. SameasExamplel 100 5.5 Comparativematerial..- 0.05 0.01 0.21 0.015 0. 100 90 7 0.05 0.01 0.25 0.015 0. Chemical oxidation, in 5% 32 2.5

chromic acid solution for 3 seconds at 90 C. Comparativematerial..- 0.05 0.01 0.25 0.015 do 100 90 8 0.07 0.02 0.29 0.013 Chemical oxidation, in 10% 45 3 potassium permanganate solution for 30 seconds at 90 0. Comparative material... 0.07 0. 02 0.29 0.013 do 100 100 9 0. 05 0.01 0.25 0. 015 Sameasin Examplel 7.5 Comparative material..- 0.05 0. 01 0.25 0.015 100 90 10 0.05 0.01 1.0 0.015 0.015 0.08 do 100 10 Comparativematerial... 0.05 0.01 0.25 0.015 0.015 0.08 do 100 90 11 0.07 0.02 0.29 0.013 0.011 0.08 1.0 Sameasin ExampleS 85 12 Comparativematerial..- 0.07 0.02 0.29 0.013 0.011 0.08 100 100 12 0.07 0.02 0.04 0.015 0.015 0.08 1.0 Same asin Example7 100 8.5 comparativematerial..- 0.07 0.02 0.25 0.015 0.015 0.08 do 100 80 13 0.06 0.01 1.0 0.021 0.009 0.05 0.05 Sameasin Exemplel 20 1.5 Comparativematerial..- 0.06 0.01 0.20 0.021 0.009 0.05 do 100 90 14 0.06 0.01 0.60 0.011 0.000 0.05 0.05 Sameasin Example2 40 2 Comparativematerial..- 0.06 0. 01 0.23 0.011 0.000 0.05 do 100 85 15 0.06 0.01 0.40 0.011 0.009 0.05 0.05 0.05 Sameasin Example7 20 1.0 Comparativematerial..- 0.06 0.01 0.23 0.011 0.009 0.05 do 100 85 16 0.06 0.01 0.40 0.011 0.009 0.05 0.05 0.10 Sameasin ExampleS 3.5 Comparativematerial... 0.06 0.01 0.23 0.011 0.009 0.05 do 100 35 1 Remarks.-Eva1uation of rust iormaton is determined by the rust formation percentage obtained by keeping the specimens in a humidity cell (J ISZ-0228) at a temperature of 49 0. 1 C. and arelative humidity What is claimed is:

1. A method for producing a steel sheet having resistance against rust formation comprising applying an oxidation treatment to a steel sheet having substantially the following composition:

not more than 0.15% of C,

not more than 0.02% of S,

not more than 0.15 of Si,

0.005 to 0.15 of P,

0.01 to 0.25% of Cu, and at least one element, in the amount indicated, selected from the group consisting of from 0.3 to 2.0 of Mn,

0.05 to 2.0% of Al,

0.01 to 0.2% of Sb,

0.05 to 2.0% of Cr,

0.001 to 0.20% of As, and

0.01 to 0.20% of Bi,

and wherein Mn +Cr+-Al2.0%,

more than 98% for 24 hours. But the rust formation of the comparative materials is one obtained when the specimens are kept in the same humid ity cell for 6 hours.

the balance being iron, by treating said sheet with an aqueous solution of chromic acid, a chromic salt or a permanganate salt to form a non-visible oxide film on the surface thereof composed mainly of 'y-Fe O 2. The method of claim 1 wherein the amount of Al is 0.05% and the amount of Cr is 0.05%.

References Cited UNITED STATES PATENTS 3,479,229 11/1969 Becker 148'-6.'35 X 3,459,538 8/1969 Teramae et a1. 75-125 3,466,234 9/ 1969 Cohen et a1. 20456 R RALPH S. KENDALL, Primary Examiner U.S. Cl. X.R. 148-62; 204-56 R 

